As one of the fields to which the present invention is applicable, a liquid crystal display (LCD) is a device that generally displays an image by injecting a liquid crystal material between an upper substrate and a lower substrate, changing orientations of liquid crystal molecules by forming an electric field between pixel electrodes and common electrodes, and adjusting the transmissivity of light according to the orientations of the liquid crystal molecules, wherein the upper substrate has common electrodes, color filters and the like formed therein, and the lower substrate has thin film transistors, pixel electrodes and the like formed therein.
Since this liquid crystal display panel is a passive element that does not voluntarily emit light, a backlight unit is necessarily installed in the liquid crystal display panel to supply light. In general, the backlight unit includes a light source supplying light; a diffuser plate or a light guide plate converting a line light source or a point light source into a surface light source; and a variety of optical films used to improve optical performances.
The optical films used in the backlight unit includes a collimating film used to improve brightness; a diffusing film having the effect of shielding optical defects in the rear of the backlight, or bright lines of the light source; a protective film used to protect the collimating film or the diffusing film from being scratched, etc.
Among them, the collimating film has lens structures arranged periodically to deflect a light path at one surface thereof. The lens structures used in the collimating film include a trigonal prism lens, a semicircular lenticular lens, micro lens, a Fresnel lens, etc.
These lens structures have functions to collimate light emitted from a light source towards the front of a display device to effectively improve brightness of the display device. However, the collimating films have problems regarding the Moire, wet-out and Newton ring phenomena, which are caused by the periodicity of the lens structures and air-gap, and also have a disadvantage in that surface defects occur on a screen due to the above-mentioned phenomena.
Also, an adhesion (blocking) phenomenon between optical films is caused while stacking a plurality of optical films with each other. However, the blocking phenomenon also causes surface defects to occur on a screen.
Therefore, in order to solve the above problems, there have been attempts to relieve the regularity of lens structures such as prism or lenticular lens. As one representative example, there has been proposed a method for relieving the regularity of a lens structure by throwing beads having a size of several micrometers to several tens micrometers to a surface of a mold having an engraved shape of a lens structure and sanding the lens structure. When the mold having an engraved shape of a lens structure is sanded with the beads, random secondary structures are further formed on the lens structure. As a result, the regularity of the lens structures is relieved to reduce the Moire phenomenon.
However, this method has problems in that it is difficult to expect a position where a secondary structure is formed due to the difficulty in controlling a position where beads are injected, and optical performances are deteriorated since beads are not injected to a concave surface of the mold due to the air turbulence, but sanded only on a convex surface of the mold. Also, the reliability of products is degraded since the reproducibility is not maintained at every sanding process. Furthermore, optical films prepared in these methods show somewhat improved Moire phenomenon, but have problems in that a collimating effect of the lens structures is deteriorated and the haze is increased due to the random formation of secondary structures. Also, this bead sanding process has its limits to suppress the wet-out phenomenon or the blocking between films.